Efficient flowing blood signal suppression is critical for accurate morphology measurements and diagnosis in magnetic resonance (MR) vessel wall imaging. Due to the complicated flow patterns in the carotid artery bifurcation, however, current black-blood (BB) imaging of the carotid bifurcation is frequently compromised by plaque-mimicking artifacts. The currently widely used BB imaging techniques include an in-flow (blood) saturation (IS) technique, and a double inversion recovery (DIR) imaging technique. The IS technique achieves BB imaging by pre-saturating the flowing blood signal before the blood enters the imaging area, and then acquiring images as the signal-suppressed blood flows through the imaging area. The blood suppression capability of the IS technique is primarily limited by the blood replenishing rate, which typically is characterized by a short preparation time.
Because of the relatively short preparation time of the IS technique compared to the DIR technique, the IS technique has been primarily used in fast imaging applications or for other occasions when a long preparation time is unacceptable. Instead of saturating the blood signal as the IS technique does, the DIR technique and its variations achieve BB imaging by inverting the out-of-slice blood signal with a 180 degree pulse and then acquiring images only when the magnetization of inflowing blood has achieved a zero-point, after delaying an appropriate inversion time (TI). The DIR technique works at a lower flow replenishing rate, since it requires the blood to be replaced after a relatively long preparation time (i.e., after a preparation duration corresponding to TI). Because of its better blood suppression capability, the DIR technique is currently widely used in vessel wall imaging applications or on other occasions when better blood suppression is desired. Both IS and DIR techniques, however, are limited by the blood replenishing rate in the through-plane direction and therefore, are both unable to avoid plaque-mimicking artifacts when recirculation occurs, or slow or stagnant flow exists.
Three-dimensional (3-D) image acquisition is of increasing interest in the black-blood imaging area due to the fact that it can provide isotropic voxel size and consequently, can facilitate the image reformation at different orientations. However, conventional imaging techniques that are typically employed for suppressing the effect of flowing blood are not well-suited for use in 3-D images. The traditional black-blood imaging techniques (IS and DIR) are based on the blood replenishing rate for a limited imaging volume. However, insufficient blood suppression will be observed in regions where there is stagnant or slow-flowing blood. This insufficiency will become more evident as a larger imaging volume is used, especially in 3-D imaging applications. Accordingly, it would be desirable to develop a better technique for black-blood imaging that is usable in 3-D applications.
To achieve sufficient blood suppression, flow-dephasing BB imaging techniques, such as a motion-sensitization driven equilibrium (MSDE) sequence, have recently been used for carotid artery vessel wall imaging. The MSDE technique utilizes a pair of flow sensitizing gradients to achieve BB imaging. The flow sensitizing gradient pair can introduce phase dispersion among moving spins, while maintaining the phase coherence of stationary spins. As has previously been reported, the flow suppression capability of the MSDE sequence is determined by the first-order moments (m1) of the flow sensitizing gradient pair. Therefore, unlike IS and DIR techniques, the MSDE technique can theoretically eliminate any slow flowing blood artifact, as long as the first order moments m1 of the gradient pair are strong enough.
One of the practical limitations of the MSDE technique is the inevitable signal loss that is caused by both the inherent T2 decay and local magnetic field (B0, B1) inhomogeneity. Specifically (based on empirical experience), marked signal loss could be observed if the m1 of the sequence is set to be large. This signal loss cannot be solely explained by an increased T2 decay. Instead, it is likely that MSDE's sensitivity to the local B1 inhomogeneity may play a key role in causing the signal loss. Therefore, a new scheme that is less sensitive to the B1 inhomogeneity would clearly be desirable.